The radio spectrum is a limited resource which must be shared among a large number of devices. The number of such devices has grown rapidly in the recent years as integrated circuit technology and standards body activities allow a rapid development of consumer grade wireless communications devices such as cordless phones and wireless networking products.
There are several methods used to assure a given frequency band in the radio frequency spectrum is efficiently shared. In general, when two radio frequency devices are transmitting at the same time on the same or overlapping frequency bands, the transmissions can interfere with each other. For example, if a radio receiver is attempting to receive a transmission from a wireless device, the reception can be successful if the signal level induced on the antenna of the radio receiver is sufficiently large compared to the level of impairments. The impairments can be thermally generated noise, distortion of the signal from non-ideal characteristics of the channel, or interference generated by other devices transmitting on or near the same frequency band. The impairment caused by other devices is referred to as interference.
In general, a signal can be received successfully with a given probability if the ratio of the received signal from a wireless device to all interference sources is kept at or above some acceptable level. This ratio is often referred to as the carrier to interference ratio (CIR).
To insure that an acceptable CIR is maintained, wireless protocols and standards often divide bands of the radio spectrum into several channels. To insure a high CIR, transmitting devices in relatively close proximity to a given device transmit on different channels.
Because only a finite number of channels are available in any band of the radio spectrum, when the number of transmitting devices exceeds the number channels available, channels are reused. For example, if a channel is shared by two wireless devices, some level of interference to the reception of the signals transmitted by the devices will be generated.
The level of the interference received by wireless devices on a same frequency band can be greatly reduced by channeling the band of the radio spectrum. The reduction in the interference level allows the overall system to maintain a higher CIR and a higher probability of successful reception of the desired signal.
Before the recent explosion of consumer grade wireless devices, frequency sharing was largely accomplished through tight control radio transmitting devices by regulatory agencies such as the Federal Communication Commission (FCC). This level of control works well when the transmitters are few in number and easily detected, such as is the case with radio and television transmitters. A simple example of channelization controlled by the FCC is the very high frequency (VHF) television transmission. The FCC imposed a transmission standard in which the country was divided into geographic regions. Although the spectrum was divided into channels 2-13, in any given geographic region only the odd or even channels are used. If the even channels are used in one region, the odd channels will be used in an adjacent region. Thus, the transmitters nearest a given region are guaranteed to be on different channels and the nearest transmitter using the same frequency will inherently be further away, generating less interference.
The control of transmitting devices in the manner described above works well when the transmitters are licensed through the regulatory agency and few in number. However, the proliferation of wireless and unlicensed consumer devices are large in number and require no licensing. It would be desirable for wireless devices and networks to take advantage of channelization and the resultant decrease in interference levels in unlicensed frequencies.